1. Field of the Invention
This invention relates to precision, high speed current sources which intermittently direct an output current into a selected current output path in response to a digital input signal. Such a current source may be utilized in many applications including precision digital to analog converters.
2. Description of the Prior Art
Digitally controlled precision current sources are commonly utilized in high speed current summing digital to analog converters. In order to prevent variations in output current caused by variations in transistor beta and base to emitter voltage (V.sub.BE), an operational amplifier and a transistor are commonly coupled together to form a feedback loop compensation circuit capable of controlling the current source transistor to maintain the current source transistor output current constant as beta and V.sub.BE vary. Those skilled in the art know that the beta and base-to-emitter voltage parameters of a bi-polar transistor vary substantially with the temperature of the transistor and also vary substantially with certain manufacturing process parameters. Those skilled in the art also know that the beta and base-to-emitter voltage of a transistor also vary somewhat with variations in the collector to base voltage of the transistor in the linear and almost-linear portions of its I.sub.C versus V.sub.CE operating characteristic. A discrete component version of a feedback controlled digital to analog converter of this type is disclosed in U.S. Pat. No. 3,685,045 and U.S. Pat. No. Re. 28,619 (Pastoriza). A monolithic version of a feedback controlled digital to analog converter is disclosed in U.S. Pat. No. Re. 28,633; U.S. Pat. Nos. 3,978,473; and 4,020,486 (Pastoriza).
Feedback controlled constant current sources of the type disclosed in the Pastoriza patents listed above correct comparatively slowly for variations in output current and require that discrete capacitors be coupled to the operational amplifier to prevent instability in the high gain feedback loop. In addition, the requirement of the Pastoriza circuits for an operational amplifier and discrete capacitors tends to make this closed loop feedback compensation system comparatively costly in terms of the cost ratio of the compensation circuit to the digital to analog converter circuitry.